This invention relates generally to a miniature motor used for audio and video equipment, etc., and more particularly to a miniature motor having a soundproofing plate to reduce noise leaking to the outside and prevent vibration from being transmitted to a printed circuit board having a control element.
FIG. 1 is a longitudinal sectional view illustrating the essential part of an example of miniature motor to which this invention is applied. In FIG. 1, reference numeral 1 refers to a case made of a metallic material, such as mild steel, formed into a bottom hollow tubular shape and having an arc segment-shaped permanent magnet 2 fixedly fitted to the inner circumferential surface thereof. Numeral 3 refers to a case cover made of a thermoplastic resin material and formed so as to be engaged with an open end of the case 1. Numeral 4 refers to a rotor consisting of an armature 5 facing the permanent magnet 2 and a commutator 6, and rotatably supported by bearings 7 and 8 each provided on the case 1 and the case cover 3.
Numeral 9 refers to a brush arm made of an electrically conductive material, formed into a plate shape, having at the free end thereof a brush 10 making sliding contact with the commutator 6 and provided inside the case cover 3. Inside the case cover 3 also provided is an input terminal 11 electrically connected to the brush arm 9 so that power is fed from an external power source to the armature 5 via the brush arms 9, the brushes 10 and the commutator 6.
Numeral 12 refers to a printed circuit board having a control element 13, provided outside the case cover 3, and adapted so as to control current and/or voltage to be fed to the armature 5. Numeral 14 refers to a soundproofing plate made of a soundproofing material, such as an elastic material including rubber and synthetic resin, or a material having vibration-damping and soundproofing effects, such as cloth, paper, etc. and provided outside the printed circuit board 12. Numeral 15 refers to a shield plate.
With the aforementioned construction, when current is fed to the armature 5, rotating force is imparted to the armature 5 existing in a magnetic field produced by the permanent magnet 2 fixedly fitted to the inner circumferential surface of the case 1 to cause the rotor 4 to rotate, driving various types of equipment connected to the rotor 4. During motor rotation, the noise produced by the sliding contact between the brushes 10 and the commutator 6, for example, is absorbed by the soundproofing plate 14 to prevent noise from leaking to the outside.
In the miniature motor having the aforementioned construction, the soundproofing plate 14 is usually formed into a ring shape having an outside diameter corresponding to the inside diameter of the open end of the case 1, and an inside diameter corresponding to the outside diameter of a part at which the case cover 3 houses the bearing 8. Since the material comprising the soundproofing plate 14 is usually sold as a hoop material having a predetermined width or a cutlength sheet having predetermined length and width, the manufacture of ring-shaped soundproofing plates 14 by punching such commercially available materials having predetermined dimensions involves a low yield.
FIG. 2 is a diagram of assistance in explaining an example where conventional soundproofing plates 14 are formed by punching a commercially available cutlength sheet material. In FIG. 2, the commercially available cutlength sheet material, which has a predetermined width, tends to yield waste areas, as shown by shaded areas in the figure, even when punching a plurality of soundproofing plates 14 while bringing each soundproofing plate as near as possible to each other. In the example shown in FIG. 2, material yield for punching soundproofing plates 14 is less than 50%, leading to failure to accomplish the effective use of resources.
When providing the soundproofing plate 14 on the case cover 3 as shown in FIG. 1 in the prior art, on the other hand, there has been a misunderstanding that the soundproofing of a miniature motor would be impossible unless the entire end face of the case cover 3 is covered with the soundproofing plate 14. An experimental study by the present Inventor revealed, however, that the soundproofing of a miniature motor can be accomplished satisfactorily by covering a required minimum area, rather than covering the entire end face of the case cover 3. It has also been made clear that by selecting an appropriate shape of the soundproofing plate 14 covering the required minimum area, a predetermined soundproofing effect can be achieved and material yield can be substantially improved.
When a control element that has to be adjusted by using a tool, such as a variable resistor, is mounted on a printed circuit board 12, for example, in the conventional type of miniature motor, a tool inserting hole has to be provided on the soundproofing plate 14 at a location corresponding to the tool inserting hole on the print circuit board 12. To this end, positioning is needed to align the tool inserting holes. This makes the assembly work of the soundproofing plate 14 troublesome, leading to lowered productivity.